In direct cooling the respective longitudinal sections in the body of metal during a conventional casting operation, liquid coolant is discharged into the ambient atmosphere of the pit below the lower end opening of the mold, and an initial longitudinal portion of a layer of liquid coolant is formed on the outer peripheral surface of the initial longitudinal section in the body of metal as the bottom block and the initial longitudinal section in the body of metal are withdrawn from the mold and lowered through the relatively upper series of levels in the pit. Then, while the bottom block and first, the initial longitudinal section in the body of metal, and then the successive additional longitudinal sections in the body of metal, are being lowered through the relatively lower series of levels in the pit during the steady state casting stage of the casting operation, an additional longitudinal portion of the layer of liquid coolant is formed on each successive additional longitudinal section in the body of metal, as the respective additional longitudinal sections in the body of metal are withdrawn from the mold through the relatively upper series of levels in the pit. Meanwhile, the liquid coolant in the initial longitudinal portion of the liquid coolant layer and in each successive additional longitudinal portion of the liquid coolant layer, flows by gravity downwardly along the surface of the body of metal through the relatively lower series of levels in the pit.
Numerous patents have been issued on the subject of direct cooling, and many of them show ways to control the process for some purpose related to varying the cooling effect of the respective longitudinal portions of the liquid coolant layer on the surface of the body of metal. See U.S. Pat Nos. 2,791,812, 3,441,079, 3,713,479, U.S. Pat. Nos. 3,623,536, 3,765,493, 4,166,495, 4,693,298, 5,040,595, 5,119,883 and U.S. Pat. No. 5,148,856 as examples. In some of the patents moreover, steps are taken to differentiate between the cooling effects to which the respective longitudinal sections in the body of metal are subjected during the butt forming stage and the steady state casting stage of the casting operation. In U.S. Pat. No. 3,441,079 to Bryson, for example, the liquid coolant is pulsed into the ambient atmosphere of the pit in a cyclical or on/off manner during the butt forming stage of the operation, to differentiate between the effects achieved during that stage and the steady state casting stage of the operation. In U.S. Pat. No. 4,351,384 to Goodrich, the initial longitudinal portion of the layer of liquid coolant is formed on the surface of the body of metal at a higher level in the relatively upper series of levels in the pit, for the butt forming stage of the operation, than are the additional longitudinal portions of the layer of liquid coolant formed thereafter for the steady state casting stage of the operation. In U.S. Pat. No. 4,166,495 to Yu, and U.S. Pat. No. 4,693,298, U.S. Pat. No. 5,040,595 and U.S. Pat. No. 5,119,883 to Wagstaff or Wagstaff et al, the mass flow rate of the liquid coolant is lowered during the butt forming stage, and then returned to a normal condition during the steady state casting stage, to differentiate between the effects achieved during the two stages. The differentiation between effects in all of these processes is achieved by making some alteration in the basic direct cooling process during the butt forming stage, and then discontinuing the alteration during the steady state casting stage. Never is it achieved in reverse, by altering the process during the steady state casting stage. Meanwhile, the steady state casting stage itself is no better at heat extraction than what the additional longitudinal portions of the layer of liquid coolant can extract from the body of metal after the alteration effected during the butt forming stage is discontinued. As a practical matter, this is a function of the per unit volume heat extraction rate of the respective additional longitudinal portions of the liquid coolant layer, and whatever improvement can be effected by increasing the rate of discharge in the liquid coolant, to increase the volume of the respective portions.